Full Flow Oil Cooler mod

Dear Friends,

I want to convert my 1986 V12 from Relief Flow to a Full Flow Oil Cooler system.
It seems to me that it would be a sensible thing to do, especially as I live in Southern France with quite warm summers and a pressure gauge that never sees relief pressures once the car is warmed up…

I did a lot of research in the archives but there are some questions still unanswered:

  1. What is the Oil Filter tread size ?
    1"x12 or 3/4x16 UNF ?

  2. What size are the Oil Hoses fittings to the Cooler ?

  3. What size are the sealed Oil Hose fittings to the Filter assembly ?

One approach is to machine the existing filter housing as per Noel [Jags+30jugs] did, but I can’t find any precise details and don’t know if it’s possible with the tools I have available.

The other approach, and probably easier, is to purchase a Universal Oil Cooler Sandwich Plate, with the added bonus of a build in 80°Thermostat, that is installed between the Filter housing and the Oil Filter, route the relief oil pipe back to the sump and use the existing oil pipes and oil cooler for the full flow set up, as per Don Neff and as Kirbert suggests in numerous posts.
But there I am confused as to witch size I should get.

And an other point, the Parts Catalogue show two different Oil Coolers, CBC2692 for Full flow and C43923 for Relief flow.
I know it has been discussed before, but has anybody established what is the difference?

Any input would be highly appreciated.

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Hi Aristides,

I looked into this some time ago when I was trying to reverse engineer the oil system.

My recollection is that the long accepted concept is wrong.

On the earlier (so called Bypass Flow) engine, bypass flow was cooled and redirected to the pump inlet (not to the sump), while engine demand was filtered and returned to the sump (without being cooled). The sump pickup pipe only needed to handle the engine demand flow rate.

On the later system (so called Full Flow), only the engine demand is filtered (and cooled) while the bypass flow is direct to the sump (bypassed directly into the sump pan). In this system, the pump inlet pipe must suck the full pump flow, hence the inlet pipe is substantially larger diameter.

The difference?
The earlier “Bypass Flow” system cools the bypassed oil.
The “Full Flow” system cools the “engine demand” oil.

In theory, according to the early literature, the pump was sized to provide 3 times the required flow. That is, it bypasses 2/3rds the flow and the engine consumes 1/3rd the flow.

So in theory, it seems to mean that the earlier system cools 2/3rds of the oil, while the later system only cools 1/3rd of the oil.

I have a theory why that might be. That is, that the engineers wanted the engine to warm up faster to meet emissions requirements and so didn’t want to cool cold bypass oil.

Also, there is confusion (in my mind anyway) as to how much relief flow is occurring, because I would assume relief flow only occurs when the relief valve is open. Which I assume it hardly ever is. I think. At least at idle. So it could well be that cooling results are quite different at idle and full revs. And it would take further analysis to figure out which is better in which circumstance.

I “think” this means that a full flow system will likely cool hot oil at idle, while the earlier one won’t. But if you are driving hard, then the earlier one might cool better.

Anyway, my point is that the names these systems are called doesn’t really reflect the actual cooling that is going on. A full flow system is not cooling the full flow.

Kind regards

PS. I guess there is also a difference in the pressure of oil flowing through the cooler. The relief flow would be low pressure, while the full flow would be at engine pressure. I think?

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This is what I have observed (with 20W50 oil):
You start in the morning, oil pressure after a couple of minutes peaks at around 60 PSI. This would be the relief pressure + the cooler’s back pressure due to the oil being cold and thick.
You then hit the highway immediately and you can cruise all day at 3.000 rpm and oil pressure would stay a bit bellow 50 PSI.

You start in the morning, but then you get stuck in traffic, oil gets really hot.
Then you hit the same highway and at the same 3.000 rpm the pressure will never rise above 42 PSI…

This to me indicates that on the second scenario you passed the threshold of relief cooling and none of the oil goes through the cooler.

Does anyone know exactly what the relief valve pressure is ?

Correct indeed, and also different coolers.
Don’t know what the difference is though.
On the other hand there were some posts on the forum of people pressurizing the relief cooler at some 180 PSI and it passed the test.

I want to try to fit the sandwich adapter and thermostat on the oil filter to experiment.
I believe that the thermostatic control will not only cool better when hot but also help the engine warm faster when cold.

I still need this info before I start, so I can order the parts in advance, if anyone knows.

  • What’s the Oil Filter thread size ?
  • What’s the Hose fittings to the Oil Cooler thread size ?

Best Regards,

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thanks MARK , i never realized that would be the logical way !

i have the early Bypass system , it always seems excellent , my pressure about 65 psi cold and drops to 55/60 hot , hot idle about 25psi.

everything seems OK been 26yrs,and counting!
thanks again for an interesting subject about V12 oiling system!
pic of my front mount trans cooler , i deleted the trans. cooling thru the radiator!

…which is puh lenty of oil pressure. I don’t see the issue…


the bean counters say lower pressure can help with MPG, DAMIFINO?

To answer my own question, the filter thread is indeed 1"x12 UNF.
Still looking for the Hose fittings to the Oil Cooler thread size…
Anyone ?


I am also looking for a new oil cooler to my DD6 -90. Direct replacements are not cheap and I might look for an aftermarket solution. SC Parts in the UK list the following specifications for the full-flow type:

“9 rows and 5/16” x 12 TPI unions. Size: 70.5 x 10 x 5 cm

Jaguar XJ12 Series III: Germany from engine no. 7P55661, all other countries less Canada and Japan from engine no. 7P56372 • XJS 5.3 V12 from VIN…126093 to 184573 (1985-92)"

Thanks, that helps !


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I’ve said the same thing (except the 2/3 vs 1/3 aspecf) several times on J-L when people dismiss the bypass system. Jag designed a positive displacement system adequate to feed 12 big ends, seven mains and the cams at idle. This inevitably means massive oversupply at high revs when you need the cooling most and the bypass works best. When cold they used the integral oil/water heat exchanger and filter to warm the oil via coolant on the first twelves (E-Types) and altered it on the sedans because the subframe got in the way of that packaging.

I would be careful with hoses and coolers (5/16 sounds very small) because I believe many coolers are only rated to circa 5 bar. My D uses dry sump full flow cooling with the factory oil rad certified to that figure. 5 bar /70 psi is plenty for the air/oil mix in the scavenge circuit (analgous to the bypass) but would be risky low for cold oil on the feed circuit, needing careful and prolonged warming up before using it in anger.

The full flow filter has to be rated at full engine pressure, more than 60psi . (probably more like 120psi.). Whilst the by-pass type would only need to be rated at less than 20psi.

Thanks for the note Norman.


Do your level best to not use abrupt 90 degree bends: if needed, use radiused ones.

A sharp 90 degree bend presents as much glow resistance, as 5-8 feet of straight hose.

True enough, but most spin-on oil filters are used in full flow applications, and the Jaguar item doesn’t appear to be any lighter construction than any other. And I don’t believe the European XJ-S models that have full flow oil systems from the factory call for a different filter.

I think Norman meant “cooler” instead of “filter”?

OK, that makes more sense. And the cooler for the full-flow model was definitely different.

I use a Setrab slim-line oil cooler, fits like a glove :slight_smile:

Can you please elaborate Tombag ?
Relief Flow or you modified it to Full Flow ?
And if so, how ?


Just changed the oilcooler, Engine is a 6.0L so no other mods needed…

Why did you make the change? Was the original cooler leaking?